Control of electron emission in cathode assemblies



Dec. 8, 1959 J. CARDELL ET AL 2,916,552

CONTROL OF ELECTRON EMISSION IN CATHODE ASSEMBLIES Filed Feb. 4, 1955 M .V JAMES "CARDELL .l F/zEos/z/CK 7T H/Lz.

T RNEV 1 l .Il .l /NvENTores United States Patent O CONTROL OF ELECTRON EMISSION IN CATHODE ASSEMBLIES James Cardell, Aubumdale, and Frederick T. Hill, Bedford, Mass., assignors to Raytheon Company, a corporation of Delaware Application February 4, 1955, Serial No. 486,258

4 Claims. (CL'313-346) This invention relates to electron emission assemblies and, particularly, to the establishment of an efficient relationship between the physical and chemical composition of an electron emitting material, on the one hand, and the physical and chemical composition of the current conducting core, or body, upon which said material is superimposed, on the other.

The invention utilizes the discovery lthat the emissivity characteristics of a cathode coating, or other electron emitting material, are radically altered by each of two conditions, one physical, the other chemical, to which such emittingl material may be subjected. The` physical factor is the type of electronic circuit into which the electron emitting element is incorporated, and the chemical factor is the degree of contamination of the core or body material.

Variations in these two factors, it has been discovered,

are of such effect upon emissivity as to require, for ex- 4 ample, the substitution of a seventy-five percent barium carbonate content in lieuiof a twenty percent barium carbonate content for the emission material when the above-described physical and chemical factors are relatively severe. Thus, while a twenty percent barium carbonate content results in most efficient emissivity when the emission element is vthe coating on a silicon-free cathode of an amplifier tube having avrelatively lowtemperature cut-olf point, with aA plate potential of, say, four volts and a heater voltage on the order of 1.7 volts, and with low-field emission, such a relatively low barium carbonate-strontium carbonate ratio does not give comparably good results when the emissionH element is a coating on an impure (silicon-bearing) `nickel cathode incorporated in a tube having a high temperature cutoff level, such as is characteristic of tubes employed in electronic computers, where the cut-ofitemperature is of such a high value as to establish a high resistance effect at the cathode interface betweenv the core and coating. For such severe duty it has been discovered that raising the barium carbonate-strontium carbonate ratio to 75/25 will provide a beneficial preponderance of barium compound to serve as a buffer agent to check the tendency of the silicon impurities of the cathode core to migrate toward the cathode interface. It is believed that this migration-checking is accomplished by4 the chemical union of barium-oxide with the migrating silicon molecules, which chemical process can proceed with maximum thoroughness only if the percentage of barium compound in the coating material is high enough to supply the chemical basis for the reaction. It has been found that a barium carbonate percentage Well above 50% (as compared with the strontium carbonate percentage), and preferably approaching 75%, is desirable to assure an effective degree of opposition to the silicon migration tendency above described. The result is a maintenance of the interface area in a relatively silicon-free condition, hence in a condition of maximum ratio.

vrice conductivity, with the resistance measurable at the interface dropping to a zero, or near-zero, value.

Other characteristics and advantages flowing from practice of the invention will be apparent upon reference to the following additional explanation, and to the accompanying drawing wherein:

Fig. 1 indicates pertinent chemical reactions, including those occurring when a high-barium coating is applied to a silicon-bearing cathode core;

Fig. 2 shows graphically the effect of increases in barium content upon interface resistance, withvand with out silicon or other impurities in the cathode core;

Fig. 3 shows a tube having, a cathode to which additional impurities may migrate as heat is applied to all tube parts; and

Fig. 4 is an enlarged sectional view of the cathode assembly of Fig. 3.

The high-barium coating material-of the present invention may be prepared, in a small-batch quantity, by weighing out 1351.5 grams of C.P. barium nitrate and 569.4 grams of C.P. strontium nitrate, and placing both materials in a vessel containing ten liters of distilledv water for solution therein. After filtering, the clear solution may be placed in a suitable reaction vessel for reaction with the proper quantities of anhydrous sodium carbonate and ammonium carbonate, under heat application, as more fully described in U.S. application Ser. No. 306,879, now Patent No. 2,703,790, filed by M. L. Anderson on August 28, 1952, and assigned to the assignee of the subject application, the reaction products being as indicated in Equationl of Fig. l. The two precipitated carbonates are then washed, filtered, dried, and prepared for use as a cathode coating having a barium-strontium ratio of /25, by molecular weight.

Due to the large preponderance of barium carbonate in the coating material, there-will be a correspondingly large preponderance of barium-oxide remaining after the evolution of the CO2 gas, in accordance with Equation 2 of Fig. l.v Accordingly, there will be suilcient bariumance 4characteristics of the interface so that the cathode current is transferred across the interface and to the coating emission surface with ,no resistance being encountered at ,the interface.- This is in sharp contrast to the high interface resistance characterizing other cathode assemblies of the kind described, differing from the low resistance assembly only in the percentage of barium carbonate entering into the barium-strontium The contrast in resistance effects appears to warrant the conclusion that the additional barium content creates the silicon migration-buffering effect above described, by which effect the interface area is freed of the silicon and other impurities, at least to a sufficient degree to reduce the interface resistance to zero, or approximately zero, even with operation of the cathode circuit at a high temperature plateau, such as characterizes tubes used in high-speed pulsing systems. It has been found that to insure checking of this silicon migration to the degree necessary to reduce the interface resistance to zero, or approximately zero, and to maintain the zero resistance Value at the interface even when the tube is subjected to constant operation at the high temperature plateaus characteristic of high-speed pulsing circuits, a barium-strontium molar ratio of at least 75/25 is required. The solid line curve of Fig. 2 (signifying use offa cathode containing silicon impurities in a high temperature cut-cti tube, or equivalent severe-duty application) shows that the interface resistance dropped to zero only when the 75/25' ratio of barium'carbonate to strontium carbonate was employed, in the particular tube tested." The broken line curve of Fig. 2(signify ing use of.a silicon-free cathode in a conventional tube operationwith a relatively low cut-off temperature) shows that the interface .resistance droppd to zero at a 50/50`ratio value, in this particular test.

Fig. 3 shows a type Vof electron discharge tube in which silicon. migration in the indirectly-heated cathode core 11 mayibesupplemented Yby `additional silicon migration into the core by way of lead-in tab 13, if the latter also contains silicon impurities, as 'it frequently does. If such a tube 'is' applied to Ythe severe duty represented by the use ofhightemperature"cut-oit tubes in computer circuitry, or the like, the effect of such high temperature will be to 'cause the electrical resistance at the cathode interface 14 (Fig. 4) to have a tendency to be objectionably high,` and this in turn will produce a corresponding 'reduction in emissivity at the surface 15 of the cathode coating, unless the coating contains a barium'V carbonate preponderance over the strontium carbonate content'in Va ratio approaching 75/25, as recommended herein; WithV sucha barium preponderance there will be anassurance 'of an 'adequate supply of barium oxide for chemical union with all of the migrating silicon, in accordance with Equation 3 of Fig. 1. Such completechemical absorption of all the migrating silicon will clearl the cathode interface of all silicon and related impurities, at least to the degree necessary to drop the interface resistance to a zero, or near-zero, value.

It will be understood, of course, that the triode tube illustrated in Fig. 3 (in which the coiled grid 16, anode 17, and shield 18 are held concentrically spaced by mica disc 12) represents merely one type of tube whose performance can be improved by embodiment of the hereindisclosedinvention therein. The invention can be applied with comparable efficiency to electron-emitting diodes, tetrodes, or any other known kind of electron dischargeI Ldevice having a cathode interface resistance problem and a cathode impurity problem such as are inherent inany tube comparable to the tube illustrated and a coating on said core comprising a mixture of oxygen-bearingv "bariu'rr1`""and *`stront'ium"compounds, in

in Fig. 3;; Likewise, the invention is not limited to the precise proportions, relationships, combinations, and proceduresfdescribed, as many equivalents will suggest them-1 which the ratio of barium to strontium is substantially seventy-tive to twenty-ve percent by molecular weight, whereby a low-resistance interface intermediate said core and said coating including the complex silicates of barium and strontium is formed, 'thereby enabling the free iiow oftelectronsffromfsaid cathode. t 1

2. In an electrondischarge'device adapted for high temperature level operation, a cathode comprising a core of conducting material having siliconimpurities therein, and a coating on said core comprising alkaline rare earth metal oxides including barium and strontium, in which the ratio of barium-toi strontium by molecular weight is in a range effective-to reduce the interface resistance between said core and said coating to a value of substantially zero, thereby enabling the free flow of electronsfrom said cathode, said interface intermediate said core and said coating including the complex silicates of said rare earth metals.'

3. In combination, an electronf discharge device adapted for hightemperature l'evel`operation, said device having a cathode comprising a'core of conducting material having silicon impurities therein, a layer of electron emissive material on said core, said layer comprising a mixture of a barium compound and a strontium compound'lwherein the barium compound'is present in a proportion substantially seventy-five percent by molecular weight, and means operative'to subject said electron discharge device to high temperature "leveloperation,

4. In an electron discharge device adaptedfor high temperature level operation, a cathodecomprisng a core of conducting 'material having silicon impurities therein, and a layer of electron emissive material on said core, said layer comprising a mixture of a barium compound and a strontium compound whereinthev ratio of barium to strontium is substantially 'seventy-tive percent to twenty-f1ve percent'by molecular weight."

References Cited in the file of this patent UNITED STATES PATENTS Great Britain June16, 1954 

1. IN AN ELECTRON DISCHARGE DEVICE ADAPTED FOR HIGH TEMPERATURE LEVEL OPERATION, A CATHODE COMPRISING A CORE OF CONDUCTING MATERIAL HAVING SILICON IMPURITIES THEREIN, AND A COATING ON SAID CORE COMPRISING A MIXTURE OF OXYGEN-BEARING BARIUM AND STRONTIUM COMPOUNDS, IN WHICH THE RATIO OF BARIUM TO STRONTIUM COMPOUNDS, IN SEVENTY-FIVE TO WTENTY-FIVE PERCENT BY MOLECULAR WEIGHT WHEREBY A LOW-RESISTANCE INTERFACE INTERMEDIATE SAID CORE AND SAID COATING INCLUDING THE COMPLEX SILICTES OF BARIUM AND STRONTIUM IS FORMED, THEREBY ENABLING THE FREE FLOW ELECTRONS FROM SAID CATHODE. 